4 ms·

> Airplanes are typically "driven" (using positive power) all throughout the flight regime, including descent and approach. No, not really. Descent in most air

> Airplanes are typically "driven" (using positive power) all throughout the flight regime, including descent and approach. No, not really. Descent in most aircraft is typically done at idle. In jet powered aircraft, final approach will be done with the engines at some speed above idle, but that is to minimize spool-up time during a go-around, rather than because the aircraft needs the thrust.

sokoloff 8y ago
The airplanes where electrics could possibly compete are piston powered and flown with power in the descent. Electrics don’t even compete well there yet (and may never as they are just as heavy on landing as takeoff). For airplanes with higher fuel burn (long haul jets), they benefit significantly from being lighter at the end of the flight. Even my piston airplane picks up about 5 knots in cruise at 1/4 tanks vs full. In jet descents, we rarely get what we prefer which is a slam dunk approach (much more fuel efficient to stay up high). Many times we’re down at 10K feet for 20+ miles because of sequencing. It’s only in the less busy airports where jets can regularly get slam dunk flight idle approaches. Even there, you probably want to use potential->kinetic rather than potential->chemical->(later) kinetic transfers because of unavoidable losses at each conversion. On the runway, using regen instead of thrust reversers or beta/reverse would be helpful, but that's a vanishingly small part of the overall flight regime.
SketchySeaBeast 8y ago
> Descent in most aircraft is typically done at idle. Went on my first flight in decades last year. The moment we started out descent really stands out to me, I remember the eerie-ness of hearing the engines slow down a ton all of a sudden and no one looking disturbed by it at all.
mrguyorama 8y ago
Idle turbo-fan engines produce enough thrust to taxi around an airport
ams6110 8y ago
Even so, the aircraft still needs to make a comfortable gradual descent. It must maintain lift as it descends, otherwise it will stall (aerodynamically) and fall out of the sky. The controlled descent takes most of the energy that you might think could be recovered by e.g. windmilling the electric fan motors as generators.
- AshleyGrant 8y ago
  I'm not sure what you're talking about with "must maintain lift as descends otherwise it will stall and fall out of the sky"... Wings stall when they exceed their critical angle of attack. A descent is inherently a low AoA maneuver. Heck, in pretty much all aircraft of decent performance, they have devices on the wings to help "spoil" the lift of the wings to help the plane descend more quickly. They're sometimes called speed brakes, sometimes called spoilers.
  - ams6110 8y ago
    Right, but you need a certain amount of forward speed relative to descent to maintain that AoA (assuming you want to maintain a fairly gradual, level descent; most passengers would find a steep nose-down descent alarming). If you scrub off all your forward speed by turning your propeller/fan into a generator to charge the batteries, you have to push the nose down to maintain AoA.

sokoloff 8y ago

The airplanes where electrics could possibly compete are piston powered and flown with power in the descent. Electrics don’t even compete well there yet (and may never as they are just as heavy on landing as takeoff). For airplanes with higher fuel burn (long haul jets), they benefit significantly from being lighter at the end of the flight. Even my piston airplane picks up about 5 knots in cruise at 1/4 tanks vs full. In jet descents, we rarely get what we prefer which is a slam dunk approach (much more fuel efficient to stay up high). Many times we’re down at 10K feet for 20+ miles because of sequencing. It’s only in the less busy airports where jets can regularly get slam dunk flight idle approaches. Even there, you probably want to use potential->kinetic rather than potential->chemical->(later) kinetic transfers because of unavoidable losses at each conversion. On the runway, using regen instead of thrust reversers or beta/reverse would be helpful, but that's a vanishingly small part of the overall flight regime.

SketchySeaBeast 8y ago

> Descent in most aircraft is typically done at idle. Went on my first flight in decades last year. The moment we started out descent really stands out to me, I remember the eerie-ness of hearing the engines slow down a ton all of a sudden and no one looking disturbed by it at all.

mrguyorama 8y ago

Idle turbo-fan engines produce enough thrust to taxi around an airport

ams6110 8y ago

Even so, the aircraft still needs to make a comfortable gradual descent. It must maintain lift as it descends, otherwise it will stall (aerodynamically) and fall out of the sky. The controlled descent takes most of the energy that you might think could be recovered by e.g. windmilling the electric fan motors as generators.

AshleyGrant 8y ago
I'm not sure what you're talking about with "must maintain lift as descends otherwise it will stall and fall out of the sky"... Wings stall when they exceed their critical angle of attack. A descent is inherently a low AoA maneuver. Heck, in pretty much all aircraft of decent performance, they have devices on the wings to help "spoil" the lift of the wings to help the plane descend more quickly. They're sometimes called speed brakes, sometimes called spoilers.
- ams6110 8y ago
  Right, but you need a certain amount of forward speed relative to descent to maintain that AoA (assuming you want to maintain a fairly gradual, level descent; most passengers would find a steep nose-down descent alarming). If you scrub off all your forward speed by turning your propeller/fan into a generator to charge the batteries, you have to push the nose down to maintain AoA.